The installation of deep foundations in the ground often require numerous steps to be effectively completed, and is very technique dependent.
Foundation elements may be constructed by many methods, with each method having its own associated difficulties and limitations with respect to the assessment of the true profile of the excavation and, consequently, the success with which the concrete/grout has been delivered and the shape of the cured material after casting. Assessment of the profile and the integrity of any foundation element can provide the engineer/contractor/owner confidence that the construction of the foundation element meets the requirements of the design of the foundation.
A bored pile or deep foundation is one where a hole is excavated using, for example, a drilling rig with a rotary powered mechanical bucket, auger, or coring barrel, in the case of a piled shaft, or a crane suspended mechanical excavating tool, either hydrofraise (reverse circulation trench cutter) or grab, in the case for a rectangular section wall. To add strength to the foundation element, a steel cage is typically lowered into the excavation and concrete is poured into the bore in a manner in which the steel is cast in place to design level, thus providing a steel reinforced structure. The excavation techniques used take into account the soil types that are being excavated and the water pressures within them. They may be excavated dry, under water, or using bentonite slurry or polymer as a retaining fluid to equalise pressures and to stabilise the ground within the excavation. There may be casing at the top of the excavation to retain any unstable material and in the case of some bored piles, a temporary casing may extend the full length to the bottom of the pile and be removed as the concrete is placed or soon afterwards.
Placement of the concrete may be performed by a tremie pipe, extending to the base of the excavation at the start of concreting and withdrawn as the concrete level rises, or by concrete pump through a pipe in a similar manner. The concrete placement may scour the base of the excavation to dislodge any debris and lift it to the surface by flotation as the concrete level rises. The concrete may be poured either directly from the top or by short pipe in the case of a dry excavation or by a combination of methods dependent on the depth of excavation.
Auger type soil displacement and soil replacement piling techniques, such as continuous flight auger/auger cast and a variety of screw type partial displacement or full displacement piles use a hollow stemmed auger or mandrel to make the pile hole. The concrete or grout is then pumped through the auger or mandrel as it is removed to form the pile. Reinforcement is placed by gravity or by pushing from the top, after the concrete or grout has reached piling platform level.
Driven cast-in-situ piles are performed with a variety of techniques but in general they utilise a steel tube with an expendable shoe to displace the soils leaving an internal void. The tube is filled with concrete which fills the void and as the tube is removed the concrete flows to the outer edges of the shape formed. The reinforcement is either placed in the tube before the concrete is poured, thus leaving it behind as the tube is removed or placed by gravity or pushing from the top after the tube is removed.
The requirement for checking that the concreting procedure has been successfully completed without soil inclusions, voids, honeycombing or other defects is paramount to ensuring good quality construction and suitability of the foundation element for the design purposes. The precise level of the concrete at any time, combined with the delivery rate information, also assists in determining the shape of the excavation as the volume of concrete placed with depth will determine the general profile of the excavation.
Instrumentation on piling rigs used for auger cast methods can give some indication of the cross-sectional area of the bore with depth as concrete/grout flow is monitored during casting (as described in U.S. Pat. No. 6,116,819), but will not typically give details of how the cross-section is finally distributed with depth or deficiencies in the pile formation.
In all types of excavation, the precise positioning of the reinforcement cage generally remains unknown after concrete/grout has been poured.
Another method of deep foundation construction is to form an injection column where grout injected under pressure through a probe tube displaces the soil under or alongside the point of injection. The procedure may also involve mixing the soil with the grout as it is injected or it may be soil mixing with the external addition of grout. Once the injection probe has reached the required depth, pressurised grout is injected as the probe rotates and is extracted forming either a continuous column up to ground level (not necessarily of the same diameter) and reinforcement bars may be placed to strengthen the load bearing capabilities and assist in the attachment of the column to the building foundations. The direction and path of the grout injection is unknown and, thus, the column shape constructed is also unknown. A minimum bearing capability based on the minimum expected column size is typically used in the foundation design, but means of assessment of the geometry of the constructed column do not exist.
A method of assessing the general profile of any deep foundation and the location of the reinforcement steel cage or bars within the foundation profile is then a valuable tool in the quality assurance of the deep foundation construction.
A variety of testing techniques have been developed to assist in the quality assurance of deep foundations so that, once installed, an assessment in terms of reliability, final geometry, and/or integrity (among other things) may be carried out. Anomalies within the structure of deep foundations may be detected by various non-destructive methods with varying degrees of efficiency, cost, ease of use, and success. Cross Hole Sonic Logging (CHSL) as described in ASTM 6760 and Sonic Integrity Testing (SIT) ASTM D5882 and summarised in the following sections a) and b) and “Thermal Integrity Profiling of Drilled Shafts”, described by Gray Mullins, DFI Journal, Vol. 4, No. 2 Dec. 2010:
a). The cross hole sonic logging method is performed by sending ultrasonic pulses through the material of the foundation element, usually, concrete from one emitter to one or more receivers generally across a horizontal path inside the reinforced concrete structure and analysing the transit time and signal strength between each pair of water filled tubes. The test requires pre-installed access/reservation tubes of a nominal 38 to 60 mm internal diameter, cast into the pile shaft parallel to one another and to its axis. The number and configuration of the tubes is determined by the size of pile and the test sequence required. This is normally taken as one tube for every 0.25 to 0.35 m of pile diameter but a minimum of three tubes is required in order to provide any worthwhile assessment.
The placement of cross hole sonic logging (CHSL) tubes within a reinforcement cage during deep foundation construction has been a testing method available for many years and a requirement in testing specifications. Until recently, it was one of the few non-destructive testing techniques which can provide an assessment along the full length of the foundation element. The joining of the CHSL tubes at the reinforcement splice levels is difficult and has been an issue of safety for the deep foundation industry with many injuries occurring when the tubes, which are often long and heavy, need to be lifted and connected which can be by welding or screw connection or just push fit and where arms and fingers need to be inserted within the reinforcement cage during connection and installation of each reinforcing cage which carries the CHSL access tubes.
An emitter lowered down one tube transmits a high frequency signal, which is sensed at some time later by the receiver in an adjacent tube generally at the same elevation. The sensors are then moved up the pile shaft in a synchronised manner until the whole shaft has been scanned. The test is repeated for each pair of tubes, allowing for the investigation of variations which might be interpreted as anomalies of varying severity both along the length and for each transit path.
The emitter and receiver are attached to PVC cable which is used to lower the sensors down the access tubes and to determine their depth. Each access tube may have its own depth encoder fitted at the top to allow accurate depth measurements to be recorded for each cable. There are several limitations of the CHSL testing method:                1. The zone of material assessment is limited to the path between access tubes;        2. The test cannot detect any anomalies outside of the reinforcement cage;        3. The access tubes have to be nominally parallel to the pile or shaft axes;        4. The tubes need to be filled with clean water prior to performing the test.        5. The access tubes must be clean internally and fully bonded to the concrete externally. Any debris, grease or joint taping may be misinterpreted as an anomaly in the structure;        6. The method is not suitable for smaller pile/shaft diameters where the percentage of the cross section due to the tubes is high;        7. Joining of the tubes at each cage splice level is difficult, time consuming and presents many safety issues;        8. Leakage of the tubes due to filling with water when the concrete is wet to assist bonding may result in washing of concrete locally around the joints causing the very anomalies the test can detect;        9. No indication of the quality of the concrete at the base is possible;        10. Assessment of concrete quality is limited as transit time changes of 20% may be expected from minor changes in position of the sensors.        11. Cost        
b). The sonic integrity test (SIT) is carried out by tapping the head of the pile with a small hand held hammer to generate an acoustic wave which spreads from the blow down through the pile. The plane waves generated are then reflected by any changes of acoustic impedance which may be discontinuities within the pile shaft or changes of soil material type and stiffness surrounding the pile shaft and the returning waves are detected by means of an accelerometer located at the pile head.
The depth from which these waves are returned is calculated by assuming the propagation velocity of the wave in the pile concrete. This method is normally referred to as the Sonic Echo method, but has been referred to by lots of other names.
A series of blows are taken and an average of the best blows obtained is used for analysis. Each test result may be plotted to show the velocity at the pile head against time/depth. Amplification of the signal with respect to time/depth may be used to enhance the clarity of the plot (and remove the influence of attenuation along the length of the element). Reflections detected at the pile head from changes of acoustic impedance down the length of the test pile, allow for detection of anomalies in the pile cross section. A change in the pile head velocity can be detected and when assessed against time/depth, can assist identifying from where along the pile shaft the reflection may have originated. The nature and degree of change of impedance at any point is a function of the magnitude of change of pile cross-section, ground conditions and wave propagation velocity (concrete quality).
Any change in the amplitude of the acceleration recorded at pile head, other than the initial hammer blow, can therefore be interpreted to indicate one of the following:                1. The pile toe.        2. A cross-sectional crack.        3. A change in cross-section (necking or bulging). This may be deliberate due to pile construction and/or a consequence of the surrounding soils.        4. Poor concrete. (Major segregation or localised voiding)        5. Soil inclusions.        6. Changes in soil/pile shaft friction.        7. A combination of any of the above factors.        
The test is useful for rapid assessment of multiple circular shafts of a relatively short length on a site after the piles have been trimmed to sound concrete. The test is limited by a length to diameter ratio, usually around 20:1 to 30:1 dependent on the soils surrounding the shaft. It is not suitable for large diameter shafts, rectangular sections such as walls or piles that are not of uniform construction such as injection columns or soil mix columns. Major anomalies and horizontal cracks may be detected but minor anomalies are difficult to detect or define and accurately assess.
c). The Thermal Integrity Profiling (TIP) technique addresses some of the issues raised above and the methods employed are described in U.S. Pat. No. 6,783,273 and U.S. Pat. No. 8,382,369 but do not allow for detailed profiling without the placement of additional reservation pipework for probe access or the permanent installation and consequent loss of expensive strings of thermal sensors due to their permanent embedment in the structure.
The objective of thermal integrity profiling is the measurement of distribution of temperature during the curing of the material, several aspects will contribute to the temperature distribution axially and radially: the composition of the mixed materials, can lead to faster/slower times to reach the peak temperature, a lower than expected peak temperature; the background temperature and density of the soil surrounding the foundation element, being part of the heat diffusion path by thermal conductivity.
Fortuitously, the absolute temperature, the maximum temperature achieved, the cause and effect is not of great significance to the assessment that may be made by this thermal profiling method as it is the relative changes, which are significant.
By analysis of the temperature distribution at some instance in time along a line parallel to the axis of the foundation element, localised variations in temperature can be detected as can general trends, which may be indicative of several factors, such as curing speed of the material of the foundation element, the composition of the in situ concrete, density, and the effect the surrounding soil/material has in terms of thermal influence on the zones measured. From variations in temperature distribution along the length or from radially equidistance/symmetrically located measurements, some first level assessment may be made to identify anomalous zones which may require more detailed investigation.
The Thermal Integrity Profiling (TIP) technique, which is performed by assessment of the distribution of temperature within the foundation element during the curing process, may be carried out either by:
1. As described in U.S. Pat. No. 6,783,273, a probe pulled from the bottom of a large diameter tube (normally a cross hole logging tube 50 mm or 60 mm OD or larger) to the top of the foundation element (or vice versa) by means of a tripod arrangement at the head of the access tube with a means of locating the sensor in the centre and fitted with a depth encoder to allow correlation of the temperature measurements with the elevation within the foundation element. The access tubes are normally dry and the probes use thermal imaging techniques requiring a relatively large diameter tube to be pre-installed in the foundation element before concrete placement. The tube is required to be dry for the infrared measurements to be possible and, as the act of pulling the infrared sensor probe in a fluid would produce some turbulence by the probe causing flow of fluid around the sensor, it would affect accurate local temperature measurements of the surrounding concrete/grout. The procedure of measuring the distribution of temperature may be carried out once or several times in the same access tube during the concrete curing process. Several reservation tubes may be installed at different locations parallel to the axis of the foundation element.
2. As described in U.S. Pat. No. 8,382,369, a fixed permanently embedded thermal string may be used in which a series of individual sensors, such as type K thermocouples may be used and attached to the reinforcement of the foundation element prior to casting. Alternative thermal strings may be made up in series using MEMS technology, thus minimizing the number of electrical connection cables required and allowing a plurality of sensors to be deployed that may be read consecutively once the concrete has been cast. The string length may be manufactured, to correspond to the cage length or standard lengths with suitable electrical connections required at the splicing elevations. Alternatively, they may be continuous and manufactured the full length of the multiple section cage and be tied to outside of the reinforcing cage as it is being inserted into the bore.
During initial curing of the concrete, the temperature sensors may be set to record at predetermined time intervals. As the concrete/grout cures the heat of hydration will increase the temperature within the foundation element, reaching a peak and then dropping as the concrete/grout cures. Detailed investigation of the temperature throughout the element may result in identification of anomalous peaks and troughs in the temperature profile. These anomalies may be further interpreted to be the result of potential defects within the foundation element. Changes in the curing rate across the foundation element may indicate the proximity of the temperature strings to the outer wall of the concrete/grout, ingress of alien material within the anticipated envelope of the foundation element, proximity of the sensor to the surrounding bore wall and even be indicative of the surrounding soil conditions.
Embodiments relate to a method and apparatus for identifying and estimating the size and/or location of anomalies in the uniformity of concrete. Specific embodiments can provide information regarding concrete and/or grout used with a piled shaft, barrette, or other load bearing concrete or grout foundation element. Specific concrete and/or grout structures that can be investigated include, for example, foundations, piles, barrettes, or other structures. Specific embodiments can utilize a string of temperature measuring sensors placed within one or more access bore(s), such as tube(s) positioned at least partially within the concrete and/or positioned proximate the concrete. Specific embodiments can utilize tube(s) attached to a reinforcement cage or framework as access bores. A variety of placement methods of the tube(s), or conduit(s), and string may be utilized. In specific embodiments, the tube(s) cast in place or plunged into wet concrete and/or grout during construction. The conduit(s) may be positioned such that longitudinal axis of the conduit(s) follow a variety of paths, such as across and/or around the base of the foundation element. Conduit(s) placed near the base of the foundation element can detect changes in the temperature profile associated with the end bearing material of the base. Significant variations in the temperature in real time, as a result of changes in the localized concrete and/or grout curing temperature due to heat generated by hydration, are affected by the geometry and density of the surrounding materials and these variations in temperature may be due to anomalies within the concrete and/or grout material. The measurements obtained may then be used to assist in the identification of existence of, size of, type of, shape of, and/or location of anomalies in the concrete and/or grout. Specific embodiments can use such information to produce a 3D representation of the temperature variations, which can ease identification of the extent and location of the anomalies.
In a preferred embodiment, the string of thermal sensors can be retrieved from the conduits in which the string is positioned during measurement, such as temperature measurements.
FIGS. 1-22 show a variety of specific embodiments having thermal coupler and conduit pipe (e.g., thermal string positioned within a conduit), and a data reader connected to the thermal strings, described below.